Azithromycin, 9-Deoxo-9a-aza-9a-methyl-9a-homoerythromycin A, is a semi-synthetic macrolide antibiotic which can be classified as a member of the second-generation erythromycin antibacterial agent. Azithromycin has the following structure (I): 
The spectrum of azithromycin's antibacterial activity has been reported by Aronoff, et al (J. Antimicrob. Chemother, 1987, 19, 275). Its mode of action has been reviewed by Retsema, et al (Antimicrob. Ag. Chemother, 1987, 31, 1939)n, and its pharmacology has been reviewed by a number of investigators (J. Antimicrob. Chemother., 1993, 31, Suppl. E, 1-198).
Three forms of Azithromycin are known. Anhydrous azithromycin is reported as an amorphous crude product (foam) in Canadian Patent 1 191 843 (example 1). It is obtained by evaporating the final solvent (e.g. chloroform) used in the process of preparation of azithromycin. It is not a crystalline product and therefore can not be made in pure form per se in commercial scale. In laboratory scale, it can be obtained in pure form by chromatography of the crude final product or by dissolving pure crystalline azithromycin mono- or dihydrate in an organic solvent and evaporating the said solvent to obtain amorphous anhydrous azithromycin.
Canadian patents 1,202,620, 1,202,619, 1,202,963 and 1,314,876 teach the process of making azithromycin monohydrate but do not claim the resulting product. Furthermore, these patents do not provide a description of the drying process (temperature or pressure). Canadian patents 1,191,843 and 1,202,963 claim azithromycin monohydrate as a new form of azithromycin. The theoretical percentage of water in azithromycin monohydrate is 2.3%. However, Canadian Patent 1,314,876 reports a value of 3.92%, and a value of 3.2% is reported in Canadian patent 1,314,876. No reference to the percentage of water is made in the other above-mentioned Canadian patents. Azithromycin monohydrate is known to be hygroscopic (see for example European Patent 298 650 B1). This is an undesirable property since it complicates formulation of azithromycin drug product and can adversely effect its stability on long term storage.
Canadian patent 1,314,876 claims azithromycin dihydrate and, in contrast to azithromycin monohydrate, a full description of the drying process used for obtaining the product is provided. Low boiling, solvents (tetrahydrofuran and hexane) are used with 3-4 equivalent moles of water to obtain the crystalline product, which is dried under vacuum at low temperatures (20-10° C.). The use of low boiling solvents for crystallisation and low temperatures for vacuum drying of the product are prescribed presumably to control the desirable amount of water that must be evaporated to afford azithromycin dihydrate. Excess loss of water, caused by higher temperature vacuum drying, could result in the formation of azithromycin monohydrate. In contrast to anhydrous azithromycin and azithromycin monohydrate, azithromycin dihydrate has desirable properties for formulation. It is crystalline and can therefore be obtained in pure form in commercial scale. It is not hygroscopic and therefore does not pose a problem during formulation or adversely effect the stability of the resulting drug product.
It is clear that anhydrous and monohydrate forms of azithromycin are not suitable for formulation. The processes referred to in Canadian Patent 1 314 876 for the preparation of azithromycin dihydrate, while producing a non-hygroscopic form of azithromycin, have a number of disadvantages:                1. Water immiscibility of the organic solvent mixture (tetrahydrofuran plus hexane) can cause problems in obtaining pure material since crystallisation processes are known to afford pure material when the anti-solvent is miscible with the solvent used to dissolve the crude product.        2. The drying process must be very carefully controlled since an increase in temperature will cause the transformation of the non-hygroscopic dihydrate to the hygroscopic monohydrate.        3. The use of low boiling point solvents is complicated by their toxicity and possibility of formation of explosive peroxide during solvent recovery.        
It has now been surprisingly found that slow addition of water to an isopropanol solution of azithromycin results in the formation of a new form of azithromycin, namely azithromycin monohydrate isopropanol clathrate of formula II: 
The physical properties of this product and the processes used for its preparation have a number of major advantages over the existing azithromycin product forms and the procedures used for their preparation.
First, azithromycin monohydrate isopropanol clathrate is crystalline and, in contrast to anhydrous azithromycin, may be obtained in pure form.
Second, azithromycin monohydrate isopropanol clathrate is not hygroscopic and, in contrast to anhydrous azithromycin and azithromycin monohydrate, may be used in formulations of the drug product as tablets or capsules with excellent stability profiles.
Third, azithromycin monohydrate isopropanol clathrate is, in contrast to azithromycin dihydrate, obtained conveniently and reproducibly by crystallisation from isopropanol water.
Fourth, in contrast to azithromycin dihydrate, azithromycin monohydrate isopropanol clathrate is obtained by crystallisation from inexpensive solvents.
Fifth, in contrast to azithromycin dihydrate, azithromycin monohydrate isopropanol clathrate is prepared from environmentally safe solvents (hexane: Class 2; isopropanol and tetrahydrofuran: Class 3, see Federal Register, Vol. 62, No. 247, 67381, Dec. 24, 1997).
Sixth, the experimental conditions are simple and applicable to large-scale production.
Seventh, the present processes are reproducible in a wide spectrum of physical conditions and consistently afford azithromycin monohydrate isopropanol clathrate with a constant ratio of azithromycin, water and isopropanol (vacuum drying at 1-10 mm Hg at 500 to 60° C. for 12 to 24 hours).
Eighth, the product generated by the processes of the present invention is highly pure.
Ninth, the processes taught in this invention afford high yields of the product within the range of 88% to 93% (first crop). The remainder of the product is conveniently recovered from the mother liquor by evaporation of isopropanol under reduced pressure.